The present invention relates to a semiconductor light emitting device such as a semiconductor laser or a light emitting diode which uses a nitride based compound semiconductor (compound semiconductor of Group III element(s) and nitrogen and the like) and is capable of emitting light in the blue type region required for an optical disk memory having a high recording density or a laser beam printer with high definition. More specifically, the present invention relates to a semiconductor light emitting device and a semiconductor laser having such a superior light emitting characteristic as to reduce a threshold current by growing a semiconductor layer having few crystalline defects and the flat surface.
A conventional light emitting diode (LED) or laser diode (LD) emitting light in a blue-emitting region has been fabricated by successively forming compound semiconductor of group III element nitrides on a sapphire substrate by Metal Organic Chemical Vapour Deposition (hereinafter referred to as MOCVD).
For example, a semiconductor laser capable of carrying out CW oscillation in a blue-emitting region is fabricated as shown in FIG. 5 by successively forming layers of group III element nitride based compound semiconductor on a sapphire substrate 21 by the MOCVD method; a GaN buffer layer 22, a contact layer 23 of an n-type GaN, an n-type clad layer 24 of Al0.12Ga0.88N, an n-type light guide layer 25 of GaN, an active layer 26 of an InGaN based (type) compound semiconductor with multiple quantum well structure, a p-type light guide layer 27 of a p-type GaN, a p-type clad layer 28 of a p-type Al0.12Ga0.88N, and a p-type contact layer 29 of a p-type GaN; etching some of the layered semiconductor layers as shown in FIG. 5 by, for example, dry etching to expose the n-type contact layer 23, and forming an n-side electrode 31 thereon and a p-side electrode 30 on the foregoing p-type contact layer 29, respectively. The portion of the p-side electrode 30 along the stripes is utilized as the light emitting part.
However, the sapphire substrate on which the nitride based compound layers are grown has considerably different lattice constant and thermal expansion coefficient from those of the nitride based compound semiconductor layers and it is difficult to get the lattice match between them. Further, the density of the threading dislocation (TD) of the nitride based compound semiconductor layers grown thereon is as high as about 1xc3x97108 cmxe2x88x922 to 1xc3x971010 cmxe2x88x922 and the dislocation density is significantly high as compared with that, 1xc3x97102 cmxe2x88x922, of compound semiconductor layers of the red-emitting type grown on GaAs substrate. In case of semiconductor lasers, if the dislocation density is especially high, the threshold current is increased, so that it is desired to lower the dislocation density than the LEDs (light emitting diode). However, other than sapphire, any alternative substrate suitable for industrial use has not been found.
On the other hand, various studies have been carried out to grow the nitride based compound semiconductor layer which is flat in a plane and small in a dislocation density, and various methods of using a off-oriented substrate, for example, a disclosure that semiconductor layers of gallium nitride based compound are grown onto a sapphire substrate of which R plane is off-oriented by 0.8xc2x0 or low as shown in Japanese Unexamined Patent Publication No. Hei 5-190903 are proposed.
As described above, though some contrivances to improve the flatness of the nitride based compound semiconductor layer and to reduce a dislocation density are performed, these are only the proposals such that, it is sufficient to be off-oriented even slightly such as not greater than 0.8xc2x0 or not greater than 1.0xc2x0 regarding the off orientation angle, and it is not minded in which crystalline axis direction the substrate should be off-oriented. Therefore, the nitride based compound semiconductor layers having the flat surface are not always attained and the semiconductor lasers having the high performance have not been commercialized.
The present invention has been performed in consideration of these circumstances, and it is an object to provide a semiconductor light emitting device having a superior characteristic of light emitting by growing the nitride based compound semiconductor layers with the degree of high flatness on a sapphire substrate.
It is another object of the present invention to provide a semiconductor laser of a structure capable of enhancing the characteristic such that a threshold current density is reduced by forming a cleavage surface finely while improving the degree of flatness by off-orienting the sapphire substrate in the case such as a semiconductor laser in which a laser beam exit edge surface is fabricated through cleavage.
The present inventors studied seriously to make a surface of the growing layer of the nitride based compound semiconductor flat and a dislocation density small by off-orienting the sapphire substrate, and therefore have found that the flatness varied widely depending on which principal plane of the sapphire substrate was off-oriented to grow the nitride based compound semiconductor layer, and the surface flatness could not be attained when the off-oriented degree was too small, as well as it was necessary to consider its direction of the tilt for leading to a final device of high performance, for example, particularly when being used as a laser diode (LD), depending on the direction of the tilt, a fine cleavage surface could not be attained and the threshold current density could not be sufficiently reduced in some cases.
Further, the present inventors have found that the surface flatness is superior by adjusting a tilt of C plane of the sapphire substrate with the tilt xcex8a on an A axis and with the tilt xcex8m on a M axis in which A and M axes are relatively orthogonal to each other and simultaneously by off-orienting the overall tilt xcex8={xcex8a2+xcex8m2}1/2 not less than 0.2xc2x0 and not more than 0.3xc2x0, and it is possible to lower the distortion of the cleavage surface and to highly reduce the threshold current (density) by raising xcex8a in taking the cleavage surface as A plane and raising xcex8m in taking the cleavage surface as M plane when being used as a laser diode (LD).
A semiconductor light emitting device in accordance with the present invention comprises; a sapphire substrate, and a laminated semiconductor portion in which nitride based compound semiconductor layers are laminated so as to constitute a light emitting layer forming portion on the sapphire substrate, wherein a C plane of the sapphire substrate has the off orientation angle having a tilt relative to an A axis and/or a M axis in such a way that
0.2xc2x0xe2x89xa6xcex8={xcex8a2+xcex8m2}1/2xe2x89xa60.3xc2x0,
wherein
0xc2x0xe2x89xa6xcex8axe2x89xa60.3xc2x0, 0xc2x0xe2x89xa6xcex8mxe2x89xa60.3xc2x0
when taking the angle tilted relative to the A axis as xcex8a and to the M axis as xcex8m, and the nitride based compound semiconductor layers are laminated onto the surface of the off-oriented C plane. Further, the laminated semiconductor portion may be laminated so as to constitute a light emitting diode structure or a laser diode structure.
Here, a nitride based compound semiconductor means a semiconductor consisting of compound of the Group III element(s) such as Ga, Al, In and the like and N, or N and the Group V elements other than N. Accordingly, this means a semiconductor consisting of N-containing compound semiconductor, in which in addition to GaN, a mixed crystal composition ratio of the Group III elements or a mixed crystal composition ratio of the Group V elements is appropriately changed, such as AlGaN based compound in which a composition ratio between Al and Ga is changeable and InGaN based compound in which a composition ratio between In and Ga is changeable.
By composing in such a manner, the surface of the growing nitride based compound semiconductor layer became highly flat, the nitride based compound semiconductor layer having a high crystallinity could be attained and the semiconductor light emitting device having a superior light emitting characteristic of such as a high light emitting efficiency could be attained. That is, while the surface roughness of the growing nitride based compound semiconductor layer became rough particularly in xcex8 smaller than 0.2xc2x0, and the surface roughness which corresponded to the height differences between the concavity and the convexity was 37 nm on average, for example, at xcex8=0.16xc2x0, the same surface roughness was 16 nm on average at xcex8=0.23xc2x0.
A semiconductor laser in accordance with the present invention is formed, in the semiconductor light emitting device as set forth in claim 1, by forming the light emitting layer forming portion so as to constitute a semiconductor laser structure, as well as by cleaving the sapphire substrate in such a way that a laser beam exit edge surface is in parallel with the A plane of the sapphire substrate, when xcex8a is larger than xcex8m, or by cleaving it in such a way that a laser beam exit edge surface is in parallel with the M plane of the sapphire substrate, when xcex8a is smaller than xcex8m.
By fabricating in this structure, the semiconductor laser, in which off-orienting hardly affects the cleavage surface, having a highly superior light emitting characteristic may be attained, while the laser diode (LD) is formed using the off-oriented substrate. Further, as for a relation which of xcex8a and xcex8m is bigger, it is preferred that either xcex8a or xcex8m to be raised is raised more than the other as far as possible because this allows the effect on the cleavage surface resulting from off-orienting to be reduced. The rate of the larger one in comparison with smaller one is preferably twice or more.
In this case, a mask layer having a plurality of opening parts which cannot be directly grown as a semiconductor layer is provided on the sapphire substrate, and the laminated semiconductor portion is formed by laminating semiconductor layers so as to constitute a semiconductor laser on a lateral growth semiconductor layer grown epitaxially laterally from the opening parts onto the mask layer.
Specifically, the opening parts are provided side by side in stripe, and the laminated semiconductor portion is formed in such a way that a current injection region in stripe form is formed along one of the opening parts in stripe form in the laminated semiconductor portion near the one of the opening parts.
Furthermore, it is possible to grow a semiconductor layer having a further higher crystallinity by forming a concave part along the direction of the opening parts in stripe form in the mask layer under a part of the laminated semiconductor portion, in which the current injection region in stripe form is formed, or by forming a space along the direction of the opening parts in stripe form between the mask layer under a part of the laminated semiconductor portion, in which the current injection region in stripe form is formed and the lateral growth semiconductor layer.